This invention relates to a category of fiber reinforced polymer (FRP) composite products and a common means of manufacturing them. Generally, it is a group of minimally abradable products in which operational coefficient(s) of friction is a significant function. Particularly, the engaging friction surfaces of these products are formed from interlocked multiple individual staple or natural length fibers and compounded with a polymer matrix. More specifically, these separable fibers are mechanically engaged by non-papermaking nonwoven textile industry methods into fibrous webs before being compounded with the polymers.
I term products which control, purposefully influence, or are dependent upon their operational coefficients of friction Friction Controlling Devices (FCD's). These include: (1) "Friction products"; those products in which coefficients of friction are maximized at minimum wear rates. That is, the retarding of relative movement between friction surfaces is the primary function of the product, as in the operation of friction brakes and clutches, transmission bands, friction bearings, and belts. Typical brakes and clutches of this nature are further illustrated in U.S. Pat. Nos. 2,733,797 and 2,733,798, but dry brakes and clutches are also included: (2) "Anti-friction products"; those products in which coefficients of friction and wear are minimized. That is, a necessary product function is to enhance relative movement between opposing surfaces, such as in journal, thrust or friction washers, bearing surfaces, bushings and other such products.
These FCD's are minimally abradable, i.e., they are wear resistant in their intended applications. That is, after an initial break in period of operation their observable wear over few short friction engagements is negligible. And in the case, such as wear plates, where the FCD must exhibit more wear than the opposing surface, it nevertheless exhibits minimal wear over extended use.
Although the divergent FCD's have generally been produced by separate means and often separate industries, it has been a little recognized fact that in spite of the opposing functions of retarding movement or enhancing it, often the main difference between friction and anti-friction materials is a small, but functionally significant, difference in coefficients of friction which can be effected by a change in the fiber, polymer matrix, or construction of a composite material. For instance, both aircraft brakes and high energy journal bearings, products of divergent functions, have been made from similar carbon or graphite fiber reinforced heat resistant polymer composites, but by different processes. The means for reducing these multiple processes to one which is more versatile and could effect the necessary composite changes would be immediately applicable to manufacturing the various products, would have obvious favorable economic advantages, and share a common friction technology.
It is an accepted concept that FRP composites perform well in FCD's. However, numerous problems have been associated in fabricating the composite materials with fibers and resins of high heat and wear resistance. Some of these problems and processes can be summarized as follows:
1. Resin saturation of wetlaid fibers and particles.
This is sheet material such as that described in U.S. Pat. Nos. 3,270,846, 3,738,901, 3,554,860, and 4,045,608 and is made on papermaking equipment. After polymer impregnation (the matrix) and curing, it is, in fact, an FRP composite, although still called "paper". This resilient material is of a specific range of porosity (interconnected void volume), produces desirably high coefficients of friction in an oil environment and is used in practically all automotive power transmission clutches.
One problem with this composite is that the papermaking process has usually necessitated the use of cotton or other fibers which decompose under high heats of friction and/or the use of asbestos fibers, an undesirable fiber to use for environmental reasons. Total replacement of these fibers has not been accomplished in any commercially available brake/transmission paper to date. These problems are detailed in: U.S. Pat. No. 2,869,973, column 1, lines 43-54; U.S. Pat. No. 3,647,722, column 1, lines 53-67, column 2, lines 2-7; and U.S. Pat. No. 3,927,241, column 1, lines 21-29. In addition, wetlaid methods generally use large quantities of water.
This inventor's attempts to replace these fibers in the papermaking process with those of staple length Kevlar (trade mark of E. I. DuPont Co.) fibers resulted in an increased amount of water and higher shear rates being necessary than is normally employed in papermaking. However, the finished sheets exhibited excellent durability under energetic friction conditions, indicating the improvements possible by utilization of this type fiber if an appropriate method of sheet formation were found. Other investigators are attempting to form Kevlar sheet and structured materials by other than wetlaying or weaving, but their results are not yet known for application to FCD's.
2. Weaving or knitting of fibers with subsequent resin impregnation.
This has been utilized in a number of bearing liner, friction clutch and brake processes, such as described in U.S. Pat. Nos. 3,730,320, 3,765,978, 4,020,226 and 4,054,337. But, weaving and knitting are time consuming and expensive processes. In addition, if an annular shape FCD is desired, the woven or otherwise sheeted materials are generally cut into rings which result in high waste of centers and trimmings. Another problem with woven fabrics is the difficulty in producing sheets of varying thicknesses. Unique three dimensional weaving patterns difficult to repeatedly change are often needed for each thickness and fiber type.
3. Various flowable molding, bulk molding, and calendaring compression techniques.
These processes fail to obtain the interlocked fiber strength or strength of continuous length rovings inherent with other processes. U.S. Pat. No. 2,553,215 is an example of how friction materials were molded before the advantage of interlocked fibers in papermaking was utilized. Bulk or particulate molding of organic materials, while applicable to some FCD's, cannot always be controlled to produce sufficient product integrity simultaneously with porosity and resilience. In addition, it has the basic limitation that the fibers and the matrix must form a good bond in order to function as a composite material. Also, tooling costs are high.
4. Spiral or helical winding or wrapping of continuous length fibers, woven cloth, or tapes with subsequent resin impregnation.
Typical examples of these methods for friction and anti-friction applications are contained in U.S. Pat. Nos. 2,901,388, 3,639,197, 3,030,248, 2,953,418, 3,870,581 3,692,375 and 3,964,807. These methods improve the waste problem associated with cutting annuli from sheets, but they generally result in a tube of laminar character and do not provide a means of achieving interlocking fiber orientation in other than the peripheral direction. This can result in delamination of the wrappings, for instance, if a thin annular device of high porosity is produced and submitted to centrifugal forces. Also, as in the case of cylindrical thrust bearings, these weak lateral bonds between fiber loops lead to decreased compressive shear strength under axially oriented loads. In addition, helical winding of fiber tapes, continuous rovings, knitted bundles of fibers, or woven cloths is also relatively slow and expensive.
In the textile industry nonwoven cards, batts, mats, slivers, and webs are common terms for the fibrous matrix of this invention. This is the product of the nonwoven carding process that often replaces woven and wetlaid paper materials and is normally used in products such as disposable diapers and surgical garments, carpeting and carpet backing, shoe liners, compressive roll covers, and abrasive floor scrubbing pads. These nonwoven staple fibers commonly are airlaid by the Rando Web carding process (on machines supplied by Rando Machine Corp., Macedon, N.Y.) or formed on conventional carding machines such as those from Davis and Furber Machine Co., N. Andover, Mass., among others. U.S. Pat. No. 3,548,461, FIG. 1, illustrates a conventional type of carding machine and U.S. Pat. No. 2,451,915 describes an air-laying carding machine. The air laying system used herein and the types of products produced differ from the air flotation system employed in forming wholly fiberglas mats. Also, they are more versatile. Unlike other methods, most existing fibers can be card processed without the use of a liquid carrying medium and usually at a faster and less expensive rate. This substantially frees the FCD producer, rather than being limited to those fibers which can, for example, best be wetlaid, to choose those fibers which best perform in the product.
These webs are commonly needle punched (I term this reorienting) on machines such as those supplied by James Hunter Machinery, N. Adams, Mass., and Oskar Dilo Kg Maschinenfabrik, Eberbach-N. West Germany. In needling of the flat sheet, a series of barbed needles are repeatedly penetrated through the carded web normal to the plane of the web. This snags generally horizontally oriented fibers and reorientes them somewhat in the vertical direction, thereby interlocking the fibers into a tensibly stronger and integral mat. Other reorienting methods use jets of air or water, for example, that method described in U.S. Pat. No. 3,391,048. Hence, a ligated "nonwoven fabric" or "needled felt" is formed although weaving, knitting or actual felting methods are not used. An illustrative example of a needle loom is described in U.S. Pat. No. 3,117,359. By the Rontex carding and needle punching method (on the Oskar Dilo cylinder needling loom), the staple fibered web is wound on a mandrel and each helical layer is needled into the preceeding ones. In other words, at least one layer or lap of nonwoven material is formed into a layered structure by rolling at least one layer or lap upon itself in a helical winding around an axis formed by the mandrel. These cylindrical forming methods are described in U.S. Pat. Nos. 3,508,307, 3,530,557, 3,540,096, 3,758,926 and 3,952,121. When an annular friction facing is prepared from the resulting cylinder by cutting perpendicularly to the cylinder's main axis a desired thickness of facing, two favorable characteristics are apparent. First, wound layers have predominantly oriented the fibers in a circumferential (or peripheral) direction which presents the fibers in an optimum fashion for circumferentially directed friction forces. Second, the radially directed needled fibers diffuse the distinctive layers and further entangle and entwine the fibers into a stronger structure.
Another method of achieving the results of needling and introducing additional fibers to mat or tubular forms is that of fiber injection, for example, the methods described in U.S. Pat. No. 3,615,967.
Resin impregnation of nonwovens is known practice to the industry, for example, see U.S. Pat. Nos. 3,953,269, 3,819,465, 3,776,779 and 4,090,986. However, resin impregnation of nonwovens with heat and friction resistant plastics and elastomers is a relatively undeveloped area of the industry and in certain instances may require specially developed saturating equipment. One supplier of such equipment is Morrison Textile Machinery, Fort Lawn, S.C.
Although it is accepted that the interfiber strength produced by fine fibrils and hydrogen bonding in the wetlaid materials is not usually duplicated by that of these non-wetlaid materials, sufficient composite integrity for FCD purposes is achieved in my invention after polymer impregnation. In this way, a tradeoff of less, but adequate, sheet strength is more than compensated for in the final product by the broader and improved raw material options and versatility of products.
Another state-of-the-art attempt to induce interfiber entanglements is the technique of heat shrinking the fibers, similar to that of a felting process. This has been suggested in producing abrasion resistant "felts" as described in U.S. Pat. No. 2,910,763, but without resin impregnation. This process, however, limits the fiber types to those which are substantially heat retractable. Whereas my process, rather than retracting the fibers, reinforces them in a polymer matrix.
Also, it is common practice in nonwovens to utilize the technique of crosslapping. In this operation an auxiliary sheet or roving is overlayed in the width direction of a length-directed travelling web. This reciprocating folding across the machine direction from a fixed position along the machine direction gives the resulting laminar mat reinforcement in the cross direction. An example of this technique is described in U.S. Pat. No. 4,042,655. It is also used to produce by similar methods to mine, a non-related product, as is described in U.S. Pat. Nos. 3,067,482 and 3,067,483.
Crosslaps or other over or underlayments on a carded fibrous form are also commonly made. These scrims, tows, mats, cloths, webs or other continuous length materials form supporting bonding or reinforcing substrates or co-layers for the carded web and may be woven or nonwoven.
Special consideration should be given to prior art involving polytetrafluoroethylene (also called "PTFE", "TFE", or TEFLON) fibers. It is generally accepted that these fibers cannot be bonded with adhesives and must be mechanically, usually by weaving, secured to other bondable fibers to adhere to a backing for use as a bearing material. Aforementioned state-of-the-art non-woven techniques have not, as well as can be determined by this inventor, been employed to entangle textile type staple length Teflon fibers into a bondable backing.